Thermoplastic molding compounds

ABSTRACT

Thermoplastic molding materials of 
     A) from 72 to 88.5% by weight of a thermoplastic styrene or α-methylstyrene polymer or copolymer, 
     B) from 10 to 20% by weight of a first graft polymer having an average particle diameter d 50  from 400 to 600 nm, having an n-butyl acrylate based rubber-elastic graft core, and a styrene or α-methylstyrene (co)polymer graft shell, 
     C) from 1.5 to 8% by weight of a second graft polymer having a bimodal particle size distribution, the average particle diameter d 50  being from 25 to 200 nm and from 350 to 550 nm, having a butadiene or isoprene polymer or copolymer based rubber-elastic graft core, and a styrene or α-methylstyrene (co)polymer graft shell, 
     D) from 0 to 50% by weight of conventional additives, 
     where the graft polymers B) and C) constitute from 11.5 to 25% by weight of components A), B) and C), and components A) to D) amount to 100% by weight.

The present invention relates to thermoplastic molding materialscontaining, based on the total weight of the molding material,

A) from 72 to 88.5% by weight of a thermoplastic polymer of, based onA),

a1) from 50 to 100% by weight of styrene or α-methylstyrene or mixturesthereof,

a2) from 0 to 50% by weight of acrylonitrile and,

a3) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers,

B) from 10 to 20% by weight of a first graft polymer having an averageparticle diameter d₅₀ from 400 to 600 nm, comprising, based on B),

b1) from 30 to 90% by weight of a rubber-elastic graft core of, based onb1),

b11) from 80 to 99.99% by weight of n-butyl acrylate,

b12) from 0.01 to 20% by weight of at least one crosslinking monomerand,

b13) from 0 to 40% by weight of one or more further monoethylenicallyunsaturated monomers, and

b2) from 10 to 70% by weight of a graft shell of, based on b2),

b21) from 50 to 100% by weight of styrene or α-methylstyrene or mixturesthereof,

b22) from 0 to 50% by weight of acrylonitrile and

b23) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers,

C) from 1.5 to 8% by weight of a second graft polymer having a bimodalparticle size distribution, the average particle diameter d₅₀ being from25 to 200 nm on the one hand and from 350 to 550 nm on the other hand,comprising, based on C),

c1) from 30 to 90% by weight of a rubber-elastic graft core of, based onc1),

c11) from 50 to 100% by weight of butadiene or isoprene or mixturesthereof, and

c12) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers, and

c2) from 10 to 70% by weight of a graft shell of, based on c2),

c21) from 50 to 100% by weight of styrene or α-methylstyrene or mixturesthereof,

c22) from 0 to 50% by weight of acrylonitrile and,

c23) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers and

D) from 0 to 50% by weight of lubricants or mold release agents,pigments, dyes, flameproofing agents, antioxidants, light stabilizers,fibrous or particulate fillers or reinforcing materials or antistaticagents or other additives, or mixtures thereof,

the sum of the graft polymers B) and C) accounting for from 11.5 to 25%by weight of the sum of the three components A), B) and C)

and the sum of the four components A) to D) being 100% by weight.

The present invention furthermore relates to the use of thethermoplastic molding materials for the production of special shapedarticles and to the special shaped articles produced from thethermoplastic molding materials.

Shaped articles of ABS (polybutadiene rubber particles, grafted withpolystyrene/acrylonitrile, in polystyrene/acrylonitrile matrix) aredistinguished by good mechanical properties, in particular—due to thelow glass transition temperature Tg of the polybutadiene—by good impactstrengths even at low temperatures.

It has been found that it is advantageous for the production of shapedarticles, in particular by injection molding process, if small and largerubber particles are present in the ABS molding material. Such moldingmaterials are described, for example, in German laid-open applicationDOS 2,427,960. Injection molded articles which have complex geometries,for example housing parts and front panels for appliances, electricswitches, switches and installation boxes, can particularlyadvantageously be produced from molding materials having such a bimodalparticle size distribution of the rubber, owing to their goodflowability.

The disadvantage of shaped articles of ABS is that their stability tolight, oxygen and other weathering influences is not completelysatisfactory, owing to the presence of double bonds in thepolybutadiene. When exposed to the weather, ABS shaped articlestherefore tend in some cases to exhibit discoloration of the surface dueto yellowing and chalking. Furthermore, the mechanical properties, inparticular the impact strength, may deteriorate.

Shaped articles of ASA (polyalkyl acrylate rubber particles, graftedwith polystyrene/acrylonitrile, in a polystyrene/acrylonitrile matrix)do not have these disadvantages since acrylic rubber contains no doublebonds sensitive to weatherings. Accordingly, shaped articles made of ASAare extremely stable to ageing and weathering. ASA molding materials aredescribed, for example, in German laid-open application DOS 1,260,135.

The disadvantage of shaped articles of ASA is that their mechanicalproperties are inadequate for some applications, in particular thelow-temperature impact strength, which is lower than that of shapedarticles made of ABS (owing to the higher Tg of the Tg of thepolyacrylate rubbers).

German laid-open application DOS 2,901,576 discloses molding materialswhich consist of an SAN matrix, or polybutadiene rubber grafted with SANand having an average particle size d₅₀ of from 200 to 450 nm, and asmall-particle polyalkyl acrylate rubber grafted with SAN and having anaverage particle size d₅₀ of from 50 to 150 nm. Such molding materialsare not completely satisfactory with regard to their mechanicalproperties and in particular their stability to weathering (tendency toyellow).

EP-A 52 732 discloses molding materials comprising an SAN matrix, acourse-particle polybutadiene rubber grafted with SAN and having anaverage particle diameter d₅₀ of from 500 to 5000 nm, and a polyalkylacrylate or polybutadiene rubber grafted with SAN and having an averageparticle diameter d₅₀ of from 50 to 180 nm on the one hand and from 250to 600 nm on the other. These molding materials, too, are not completelysatisfactory with regard to their stability to weathering.

It is an object of the present invention to remedy the disadvantagesdescribed. In particular, it is intended to provide molding materialshaving good flowability, from which shaped articles having goodmechanical properties and sufficient stability to ageing and toweathering can be produced, in particular shaped articles having abalanced ratio of impact strength and a tendency to yellow.

We have found that this object is achieved by the thermoplastic moldingmaterials defined at the outset. We have also found the use of themolding materials for the production of special shaped articles, andspecial shaped articles consisting of the molding materials.

Component A) of the novel molding material is present in an amount offrom 72 to 88.5, preferably from 73 to 85, particularly preferably from75 to 83, % by weight, based on the sum of components A), B), C) and D).Component A) is a thermoplastic polymer which consists of

a₁) from 50 to 100, preferably from 55 to 95, particularly preferablyfrom 60 to 85, % by weight of styrene or α-methylstyrene or mixturesthereof,

a₂) from 0 to 50, preferably from 5 to 45, particularly preferably from15 to 40, % by weight of acrylonitrile and

a₃) from 0 to 50, preferably from 0 to 40, % by weight of one or morefurther monoethylenically unsaturated monomers,

the percentages in each case based on component A). The monomers statedfurther below for component b13) are suitable as further monomers a3),the monomers a1) and a2) of course being excluded from the list of themonomers b13).

The polymers A) which, owing to their main component styrene andacrylonitrile, are generally also referred to as SAN polymers, are knownand some of them are also commercially available. They have, as a rule,a viscosity number VN (determined according to DIN 53 726 at 25° C.,0.5% by weight dimethylformamide) of from 40 to 160 ml/g, correspondingto a weight average molecular weight of from about 40000 to 2000000.They are obtained in a known manner by mass, solution, suspension,precipitation or emulsion polymerization. Details of these processes aredescribed, for example in Kunststoffhandbuch, published by R. Vieweg andG. Daumiller, vol. V “Polystyrol”, Carl-Hanser-Verlag Munich 1969, page118 et seq.

Component B) of the molding material is present in an amount of from 10to 20, preferably from 12 to 18, particularly preferably from 12.5 to17, % by weight, based on the sum of the components, A), B), C) and D).

Component B) is a first graft polymer having an average particlediameter d₅₀ of from 400 to 600 nm, comprising, based on B),

b1) from 30 to 90, preferably from 40 to 80, particularly preferablyfrom 45 to 75, % by weight of a rubber-elastic graft core b1) and

b2) from 10 to 70, preferably from 20 to 65, particularly preferablyfrom 25 to 60, % by weight of a graft shell b2).

The graft core b1) is composed of, based on b1),

b11) from 80 to 99.99, preferably from 85 to 99.5, particularlypreferably from 90 to 99, % by weight of n-butyl acrylate,

b12) from 0.001 to 20, preferably from 0.5 to 10, particularlypreferably from 1 to 5, % by weight of at least one crosslinking monomerand,

b13) from 0 to 40, preferably from 0 to 20, particularly preferably from0 to 10, % by weight of one or more further monoethylenicallyunsaturated monomers.

Crosslinking monomers b12) are bi- or polyfunctional comonomers, forexample butadiene and isoprene, divinyl esters of dicarboxylic acids,for example of succinic acid and adipic acid, diallyl and divinyl ethersof bifunctional alcohols, for example of ethylene glycol and ofbutane-1,4-diol, diesters of acrylic acid and methacrylic acid with thestated bifunctional alcohols, 1,4-divinylbenzene and triallyl cyanurate.The acrylate of tricyclodecenyl alcohol of the formula

which is known under the name dihydrodicyclopentadienyl acrylate, andthe allyl esters of acrylic acid and of methacrylic acid areparticularly preferred.

Component b1) of the molding materials may furthermore contain, at theexpense of the monomers b11) and b12), further monomers b13) which varythe mechanical and thermal properties of the core within a certainrange. Examples of such monoethylenically unsaturated comonomers are:

vinylaromatic monomers, such as styrene, styrene derivatives of thegeneral formula I

where R¹ and R² are each hydrogen or C₁-C₈-alkyl;

methacrylonitrile, acrylonitrile;

acrylic acid, methacrylic acid, and dicarboxylic acids such as maleicacid and fumaric acid, and the anhydrides thereof, such as maleicanhydride;

Monomers having functional nitrogen groups, such as dimethylaminoethylacrylate, diethylaminoethyl acrylate, vinylimidazol, vinylpyrrolidone,vinylcaprolactam, vinylcarbazol, vinylaniline and acrylamide;

C₁-C₁₀-alkyl esters of acrylic acid and of methacrylic acid, such asmethyl(meth)acrylate, ethyl(meth)acrylate, n-propyl(meth)acrylate,isopropyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate,sec-butyl(meth)acrylate, tert-butyl(meth)acrylate,ethylhexyl(meth)acrylate and hydroxyethyl acrylate;

aromatic and araliphatic esters of acrylic acid and methacrylic acid,such as phenyl acrylate, phenyl methacrylate, benzyl acrylate, benzylmethacrylate, 2-phenylethyl acrylate, 2-phenylethyl methacrylate,2-phenoxyethyl acrylate and 2-phenoxyethyl methacrylate;

N-substituted maleimides, such as N-methyl-, N-phenyl- andN-cyclohexylmaleimide;

unsaturated ethers, such as vinyl methyl ether

and mixtures of these monomers.

The graft shell b2) is composed of, based on b2),

b21) from 50 to 100, preferably from 55 to 95, particularly preferablyfrom 60 to 90, % by weight of styrene or α-methylstyrene or mixturesthereof,

b22) from 0 to 50, preferably from 5 to 45, particularly preferably from10 to 40, % by weight of acrylonitrile, and

b23) from 0 to 50, preferably from 0 to 40, particularly preferably from0 to 20, % by w eight of one or more further monoethylenicallyunsaturated monomers.

Suitable further monomers b23) are the monomers stated further aboveaccording to b13), the monomers b21) and b22) of course being excludedfrom the list of the monomers b13).

The graft polymers B) are obtainable in a manner known per se,preferably by emulsion polymerization at from 30 to 80° C. Examples ofsuitable emulsifiers for this purpose are alkali metal salts ofalkenesulfonic or alkylarylsulfonic acids, alkyl sulfates, fatty alcoholsulfonates, salts of higher fatty acids of 10 to 30 carbon atoms,sulfosuccinates, ether sulfonates or resin soaps. The alkali metal saltsof alkanesulfonates or fatty acids of 10 to 18 carbon atoms arepreferably used.

For the preparation of the dispersion, water is preferably used in anamount such that the prepared dispersion has a solids content of from 20to 50% by weight.

Preferred polymerization initiators are free radical initiators, forexample peroxides, preferably peroxosulfates (for example potassiumpersulfate) and azo compounds, such as azobisisobutyronitrile. However,redox systems may also be used, in particular those based onhydroperoxides, such as cumyl hydroperoxide.

Molecular weight regulators, e.g. ethylhexyl thioglycolate, tert-dodecylmercaptan, terpinols and dimeric α-methylstyrene, may furthermore bepresent.

In order to maintain a constant pH, which is preferably from 6 to 9,buffer substances, such as Na₂HPO₄/NaH₂PO₄ or sodium bicarbonate, may bepresent.

Emulsifiers, initiators, regulators and buffer substances are used inconventional amounts, so that further information in this respect isunnecessary.

The graft core can also particularly preferably be prepared bypolymerizing the monomers b1) in the presence of a finely divided rubberlatex (i.e. seed latex polymerization procedure).

In principle, it is also possible to prepare the graft core b1) by aprocess other than emulsion polymerization, for example by mass orsolution polymerization, and subsequently to emulsify the polymersobtained. Microsuspension polymerization is also suitable, oil-solubleinitiators, such as lauroyl peroxide and tert-butyl perpivalate,preferably being used. The relevant processes are known.

The preparation of the graft shell b2) can be carried out under the sameconditions as the preparation of the graft core b1), and the shell b2)can be prepared in one or more process steps. For example, first styreneor α-methylstyrene alone and then styrene and acrylonitrile can bepolymerized in two successive steps. Further details of the preparationof the graft polymers B) are described in German laid-open applicationsDOS 1,260,135 and DOS 3,149,358.

The reaction conditions are matched with one another in a manner knownper se so that the polymer particles B) have a uniform diameter d₅₀ from400 to 600 nm, in particular from 450 to 550 nm.

Component C) of the molding materials is present in an amount of 1.5 to8, preferably from 2 to 7, particularly preferably from 2.5 to 6, % byweight, based on the sum of the components A), B), C) and D).

Component C) is a second graft polymer have a bimodal particle sizedistribution, the average particle diameter d₅₀ being from 25 to 200 nmon the one hand and from 350 to 550 nm on the other hand, comprising,based on C),

c1) from 30 to 90, preferably from 35 to 85, particularly preferablyfrom 40 to 75, % by weight of a rubber-elastic graft core c1) and

c2) from 10 to 70, preferably from 15 to 65, particularly preferablyfrom 25 to 60, % by weight of a graft shell c2).

The graft core c1) is composed of, based on c1),

c11) from 50 to 100, preferably from 80 to 100, particularly preferablyfrom 100, % by weight of butadiene or isoprene or mixtures thereof and

c12) from 0 to 50, preferably from 0 to 20, particularly preferably 0, %by weight of one or more further monoethylenically unsaturated monomers.

The graft shell c2) is composed of, based on c2),

c21) from 50 to 100, preferably from 55 to 95, particularly preferablyfrom 60 to 90, % by weight of styrene, α-methylstyrene or mixturesthereof,

c22) from 0 to 50, preferably from 5 to 45, particularly preferably from10 to 40, % by weight of acrylonitrile and

c23) from 0 to 50, preferably from 0 to 40, particularly preferably from0 to 20, % by weight of one or more further monoethylenicallyunsaturated monomers.

Suitable further monomers c23) are the monomers stated further above forcomponent b13).

The graft shell c2) is accordingly defined as described under b2).However, it is quite possible for the graft shells b2) and c2) to differwith regard to the type and amount of the monomers used.

The statements made further above in connection with the preparation ofthe graft polymers B) are as a rule applicable to the preparation of thegraft polymers C), with the exception of the particle diameter.

The reaction conditions are matched with one another so that the polymerparticles C) have a bimodal particle size distribution, i.e., the sizedistribution having two maxima. The average particle diameters d₅₀ arefrom 25 to 200 am, preferably from 60 to 140 nm, particularly preferablyfrom 70 to 120 nm, on the one hand, and from 350 to 550 nm, preferablyfrom 400 to 500 nm, particularly preferably from 430 to 470 nm, on theother hand. Details are described in German laid-open application DOS2,427,960.

For example, the following procedure may be adopted: the monomers c1)which form the core are polymerized to a conversion of, usually, atleast 90%, preferably at least 95%, based on the monomers used. Thisconversion is reached, as a rule, after from 4 to 20 hours.

The rubber latex obtained in this manner has an average particle sized₅₀ of not more than 200 nm and a narrow particle size distribution(virtually monodisperse system).

In the second stage, the rubber latex is agglomerated. This is done, asa rule, by adding a dispersion of an acrylate polymer. Preferably,dispersions of copolymers of acrylates of alcohols of 1 to 4 carbonatoms, preferably of ethyl acrylate, with from 0.1 to 10% by weight, ofmonomers forming water-soluble polymers, e.g. acrylic acid, methacrylicacid, acrylamide or methacrylamide, N-methylolmethacrylamide orN-vinylpyrrolidone, are used. A copolymer of 96% of ethyl acrylate and4% of methacrylamide is particularly preferred. The agglomeratingdispersion can, if desired, also contain a plurality of the statedacrylate polymers.

The concentration of the acrylate polymers in the dispersion should ingeneral be from 3 to 40% by weight. In the agglomeration, from 0.2 to20, preferably from 1 to 5, parts by weight of the agglomeratingdispersion are used per 100 parts by weight of the rubber latex,calculated in each case relative to solids. The agglomeration is carriedout by adding the agglomerating dispersion to the rubber. The rate ofthe addition is usually not critical, the addition generally lasting forfrom about 1 to 30 minutes at from 20 and 90° C., preferably from 30 and75° C.

Under the stated conditions, only some of the rubber particles areagglomerated, resulting in a bimodal distribution. After theagglomeration, in general more than 50, preferably from 75 to 95, % ofthe particles (numerical distribution) are present in thenonagglomerated state. The partially agglomerated rubber latex obtainedis relatively stable so that it can be directly stored and transportedwithout coagulation occurring.

The core c1) is grafted with the graft shell c2) in a manner known perse, for example in the same system as the preparation of the graft corec1).

If ungrafted monomers are formed from the monomers b2) and c2) in thegrafting of the cores b1) and c1), respectively, these amounts, which asa rule are less than 10% by weight of b2) and c2), are assigned to themass of the components B) and C), respectively.

According to the invention, the sum of the graft polymers B) and C) isfrom 11.5 to 25, preferably from 15 to 24, particularly preferably from16 to 22, % by weight, based on the sum of the three components A), B)and C).

Component D) of the molding materials is present in an amount of from 0to 50, preferably from 0 to 20, particularly preferably from 0 to 10, %by weight, based on the sum of the components A), B), C) and D).

Component D) comprises lubricants or mold release agents, pigments,dyes, flameproofing agents, antioxidants, light stabilizers, fibrous andpulverulent fillers or reinforcing materials or antistatic agents andother additives or mixtures thereof.

Suitable lubricants and mold release agents are, for example, stearicacids, stearyl alcohol, stearates or stearamides, as well as siliconeoils, montan waxes and those based on polyethylene and polypropylene.

Pigments are, for example, titanium dioxide, phthalocyanines,ultramarine blue, iron oxides or carbon black, as well as the classconsisting of the organic pigments.

Dyes are to be understood as meaning all dyes which can be used fortransparent, semitransparent or nontransparent colouring of polymers, inparticular those which are suitable for coloring styrene copolymers.Such dyes are known to a person skilled in the art.

For example, the halogen-containing or phosphorus-containing compoundsknown to a person skilled in the art, magnesium hydroxide and otherconventional compounds or mixtures thereof may be used as flameproofingagents.

Suitable antioxidants (heat stabilizers) are, for example, stearicallyhindered phenols, hydroquinones, various substituted members of thisgroup and mixtures thereof. They are commercially available, for exampleas Topanol® or Irganox®.

Suitable light stabilizers are, for example, various substitutedresorcinols, salicylates, benzotriazoles, benzophenones and HALS(Hindered Amine Light Stabilizers), as commercially available, forexample, as Tinuvin®.

Examples of fibrous or pulverulent fillers are carbon fibers or glassfibers in the form of woven glass fabrics, glass mats or glass rovings,chopped glass, glass beads and wollastonite, particular preferably glassfibers. If glass fibers are used, they may be provided with a size andan adhesion promoter to improve the compatibility with the blendcomponents. The glass fibers can be incorporated both in the form ofshort glass fibers and in the form of rovings.

Suitable particulate fillers are carbon black, amorphous silica,magnesium carbonate (chalk), powdered quartz, mica, bentonite, talc,feldspar and in particular calcium silicates, such as wollastonite andkaolin.

Examples of suitable antistatic agents are amine derivatives, such asN,N-bis(hydroxyalkyl) alkylamines, N,N-bis(hydroxyalkyl) alkyleneamines,polyethylene glycol esters and glyceryl mono- and distearates andmixtures thereof.

Individual additives are used in the usual amounts in each case, so thatfurther information in this connection is unnecessary.

The novel molding materials can be prepared by mixing methods known perse, for example with melting in an extruder, Banbury mixer, kneader,roll mill or calendar. However, the components can also be used cold andthe powder mixture or mixture consisting of granules is not melted orhomogenized until the processing stage.

The molding materials can be converted into moldings of all kinds, inparticular injection moldings, very particularly those having complexgeometries, such as housing parts or electric switches. The novelmolding materials can furthermore be used for the production of frontpanels for household appliances, for the production of panels, switchesor covers in the electrical sector or for the production of installationboxes for outdoor electrical apparatuses. The shaped articles aredistinguished by a combination of high stability to weathering (littletendency to yellow) and good mechanical properties (high impactstrength).

The stated average particle size d is the weight average particle sizeas determined by means of an analytical ultracentrifuge according to themethod of W. Scholtan and H. Lange, Kolloid-Z. und Z.-Polymere 250(1972) 782-796. The ultracentrifuge measurement gives the integral massdistribution of the particle diameter of samples. From this it ispossible to determine the percentage by weight of the particles whichhave a diameter equal to or less than a certain size.

The d₁₀-value indicates the particle diameter at which 10% by weight ofall particles have a smaller diameter and 90% by weight a largerdiameter. Conversely, it is true for the d₉₀-value that 90% by weight ofall particles have a smaller diameter, and 10% by weight a largerdiameter, than the diameter which corresponds to the d₉₀-value. Theweight average particle diameter d₅₀ or volume average particle diameterD₅₀ indicates the particle diameter at which 50% by weight or % byvolume, respectively, of all particles have a larger diameter and 50% byweight or % by volume, respectively, have a smaller particle diameter.d₁₀-, d₅₀- and d₉₀-values characterize the width Q of the particle sizedistribution, where Q=(d₉₀−d₁₀)/d₅₀. The smaller Q, the narrower is thedistribution.

EXAMPLES Preparation of a Component A

Copolymer of Styrene and Acrylonitrile

A copolymer of 65% by weight of styrene and 35% by weight ofacrylonitrile was prepared by the continuous solution polymerizationmethod as described in Kunststoff-Handbuch, publishers R. Vieweg and G.Daumiller, vol. V “Polystyrol”, Carl-Hanser-Verlag Munich 1969, pages122 to 124. The viscosity number VN (determined according to DIN 53 726at 25° C., 0.5% by weight in dimethylformamide) was 64 ml/g.

Preparation of a Component B

First Graft Polymer of Crosslinked Poly-n-butylacrylate (core) andPolystyrene/acrylonitrile(shell)

A mixture of 98 g of n-butyl acrylate and 2 g ofdihydroxydicyclopentadienyl acrylate and, separately therefrom, asolution of 1 g of sodium C₁₂-C₁₈-paraffinsulfonate in 50 g of waterwere added, in the course of 4 hours at 60° C., to a mixture of 3 g of apolybutyl acrylate seed latex, 100 g of water and 0.2 g of potassiumpersulfate. The polymerization was then continued for a further 3 hours.The average particle diameter d₅₀ of the resulting latex was 430 nm, theparticle size distribution being narrow (Q=0.1). The solids content was40%.

150 g of this latex were mixed with 60 g of water, 0.03 g of potassiumpersulfate and 0.05 g of lauroyl peroxide, after which first 20 g ofstyrene and then, in the course of a further 4 hours, a mixture of 15 gof styrene and 5 g of acrylonitrile were grafted onto the latexparticles in the course of 3 hours at 65° C. Thereafter, the polymer wasprecipitated with a calcium chloride solution at 95° C., isolated,washed with water and dried in a warm air stream. The degree of graftingof the polymer was 35% and the particles had an average diameter d₅₀ of510 nm.

The seed polymer initially used was prepared by the process of EP-B 6503(column 12, line 55, to column 13, line 22) by polymerization of n-butylacrylate and tricyclodecenyl acrylate in aqueous emulsion and had asolids content of 40%.

Preparation of a Component C

Second Graft Polymer of Polybutadiene (core) andPolystyrene/acrylonitrile (shell)

Preparation of the Polybutadiene Emulsion

0.5 g of tert-dodecyl mercaptan and 16.6 g of butadiene were added to amixture of 150 g of water, 1.2 g of sodium C₁₂-C₁₈-paraffinsulfonate,0.3 g of potassium persulfate, 0.3 g of sodium bicarbonate and 0.15 g ofsodium pyrophosphate at 65° C.

After 1 hour, a further 83.3 g of butadiene were metered in over 5hours. 5 hours after the end of the butadiene addition, 0.5 g oftert-dodecyl mercaptan was added. After a total reaction time of 19hours and a conversion of 96%, based on butadiene, the solids contentwas 39%. The particle size distribution was as follows: d₁₀: 60 nm, d₅₀:80 nm, d₉₀: 105 μm.

Agglomeration and Grafting

255 g of the polybutadiene emulsion were diluted at 65° C. with 74 g ofwater, and 30 g of an aqueous dispersion (solids content 10% by weight,based on this dispersion) of a copolymer of 96% by weight of ethylacrylate and 4% by weight of methacrylamide were added. After theagglomeration, about 80% by number of the polybutadiene particles werein the nonagglomerated state. A broad bimodal particle size distributionwas present: d₁₀: 79 nm, d₅₀: 238 nm, d₉₀: 323 nm.

0.13 g of potassium persulfate (as an aqueous 3% strength by weightsolution), 0.02 g of tert-dodecyl mercaptan and 11 g of a mixture of 70%by weight of styrene and 30% by weight of acrylonitrile were added at70° C. to the resulting emulsion. After 10 minutes, a mixture of 39 g ofstyrene, 17 g of acrylonitrile and 0.1 g of tert-dodecylmercaptan wasadded in the course of 2 ¾ hours, the temperature increasing to 75° C.After the end of the monomer addition, the reaction was allowed tocontinue for a further hour. Thereafter, the polymer was precipitatedwith a calcium chloride solution at 95° C., isolated, washed with waterand dried in a warm air stream.

Component D

4 parts by weight, based on 100 parts by weight of the sum of componentsA), B) and C), of titanium dioxide were used as white pigment.

Preparation of the Blends and of the Moldings

Components A), B), C) and D) were thoroughly mixed in a conventionalextruder of the type ZSK 30, from Werner and Pfleiderer, at 250° C. andextruded and granulated.

The granules were injection molded at a melt temperature of 220° C. anda mold temperature of 60° C. to give standard small bars (cf. DIN 53453). Furthermore, circular discs of 6 cm diameter and 0.2 cm thicknesswere injection molded at a melt temperature of 250° C. and a moldtemperature of 60° C.

Investigation of the Moldings

The impact strength test for determining the impact strengths werecarried out on the standard small bars according to DIN 53 453.

To determine the stability to weathering, standard small bars wereexposed for 48 and 168 hours in a Xenotest 1200 CPS UV exposure unitaccording to DIN 53 387 and the impact strength test was then carriedout according to DIN 53 453, the unexposed side being subjected toimpact.

To determine the tendency to yellow, the yellowness indexes according toDIN 6167 (January 1980) were determined for the circular discs afterexposure for 48 hours and 336 hours. The colour measurement was carriedout according to DIN 53 236, method B (January 1983). The measurementgeometry was 45°/0°, illuminant/standard observer: D65/10°.

The difference in yellowness index is to be understood as meaning thedifference from the smallest measured yellowness index of the respectiveexposure series.

The mixing ratios in the polymer blends and the properties of themoldings produced from them are shown in the table below.

TABLE Example 1 2V 3V 4V Composition [% by weight]: A 83.2 83.2 66.771.0 B 13.8 16.8 17.5 — C 3.0 — 15.8 29.0 D [parts by weight¹⁾] 4 4 4 4Properties: Impact strength a_(n) (25° C.), [kJ/m²] before weathering 6329 n.d. n.d. after weathering for 48 h 49 31 n.d. 8 after weathering for168 h 28 30 9 6 Difference in yellowness index after exposure for 48 h 00.2 0.6 0.9 after exposure for 336 h 3.1 0.4 7.5 10.3 ¹⁾based on 100parts by weight of the sum of components A), B) and C). n.d. notdetermined

The shaped articles obtained from novel molding materials (Example 1)show an optimum of high impact strength before weathering, high impactstrength after weathering and a low tendency to yellow after exposure.

The difference in the yellowness index of shaped articles containingnovel amounts of the bimodal polybutadiene graft rubber (component C)is, after exposure, slightly higher than the difference in theyellowness index of polybutadiene-free shaped articles (Comparativeexample 2V and Example 1). However, the impact strength of shapedarticles produced from novel molding materials prior to weathering ismore than twice as high and, after weathering for 168 hours, is stilljust as good as the impact strength of polybutadiene-free moldingmaterials.

Accordingly, the addition of small amounts of polybutadiene graft rubberconsiderably increases the toughness at room temperature.

Shaped articles produced from molding materials having a higher content,not according to the invention, of component C) (Comparative Examples 3Vand 4V) have, after weathering, a considerably lower impact strength anda substantially higher difference in the yellowness index than shapedarticles produced from the novel molding materials.

Although the novel molding materials contain considerably less graftrubber (sum of B) and C)) (Example 1) than Comparative Example 4V, theimpact strength after weathering is considerably higher.

We claim:
 1. A thermoplastic molding material containing, based on thetotal weight of the molding material, A) from 72 to 88.5% by weight of athermoplastic polymer of, based on A), a₁) from 50 to 100% by weight ofstyrene or α-methylstyrene or mixtures thereof, a₂) from 0 to 50% byweight of acrylonitrile and a₃) from 0 to 50% by weight of one or morefurther mono-ethylenically unsaturated monomers, B) from 10 to 20% byweight of a first graft polymer having an average particle diameter d₅₀from 400 to 600 nm, consisting of, based on B), b₁) from 30 to 90% byweight of a rubber-elastic graft core of, based on b₁), b₁₁) from 80 to99.99% by weight of n-butyl acrylate, b₁₂) from 0.01 to 20% by weight ofat least one crosslinking monomer selected from the group consisting ofdihydrodicyclopentadienyl acrylate, allyl esters of acrylic acid andally esters of methacrylic acid, and, b₁₃) from 0 to 40% by weight ofone or more further monoethylenically unsaturated monomers, and b₂) from10 to 70% by weight of a graft shell of, based on b₂), b₂₁) from 50 to100% by weight of styrene or α-methylstyrene or mixtures thereof, b₂₂)from 0 to 50% by weight of acrylonitrile, and b₂₃) from 0 to 50% byweight of one or more further monoethylenically unsaturated monomers, C)from 1.5 to 8% by weight of a second graft polymer having a bimodalparticle size distribution, the average particle diameter d₅₀ being from25 to 200 nm on the one hand and from 350 to 550 nm on the other hand,comprising, based on C), c₁) from 30 to 90% by weight of arubber-elastic graft core of, based on c₁), c₁₁) from 50 to 100% byweight of butadiene or isoprene or mixtures thereof, and, c₁₂) from 0 to50% by weight of one or more further monoethylenically unsaturatedmonomers, and, c₂) from 10 to 70% by weight of a graft shell of, basedon c₂), c₂₁) from 50 to 100% by weight of styrene or α-methylstyrene ormixtures thereof, c₂₂) from 0 to 50% by weight of acrylonitrile and,c₂₃) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers and D) from 0 to 50% by weight of lubricants ormold release agents, pigments, dyes, flameproofing agents, antioxidants,light stabilizers, fibrous or particulate fillers or reinforcingmaterials or antistatic agents or other additives, or mixtures thereof,the sum of the graft polymers B) and C) accounting for from 11.5 to 25%by weight of the sum of the three components A), B) and C), and the sumof the four components A) to D) being 100% by weight.
 2. A housing partor front panel for household appliances, a panel, switch or cover in theelectrical sector or an installation box for outdoor electricalapparatuses obtained from a thermoplastic molding material as claimed inclaim
 1. 3. The molding material defined in claim 1, wherein the amountof the thermoplastic polymer (A) is of from 73 to 85% by weight, basedon the sum of the four components A) to D).
 4. The molding materialdefined in claim 1, wherein the amount of the first graft polymer (B) isof from 12 to 18% by weight, based on the sum of the four components A)to D).
 5. The molding material defined in claim 1, wherein the amount ofthe second graft polymer (C) is of from 2 to 7% by weight, based on thesum of the four components A) to D).
 6. The molding material defined inclaim 1, wherein the sum of the graft copolymers (B) and (C) amount tofrom 15 to 24% by weight, based on the sum of components (A) to (C). 7.The molding material defined in claim 1, wherein the sum of the graftcopolymers (B) and (C) amount to from 16 to 22% by weight, based on thesum of components (A) to (C).
 8. The molding material defined in claim1, wherein the thermoplastic polymer (A) consists of a₁) from 55 to 95%by weight of styrene or α-methylstyrene or mixtures thereof, a2) from 5to 45% by weight of acrylonitrile and a₃) from 0 to 40% by weight of oneor more further monoethylenically unsaturated monomers.
 9. The moldingmaterial defined in claim 1, wherein the second graft polymer (C)comprises, based on (C), from 35 to 85% by weight of the rubber-elasticgraft core (c₁), and from 15 to 65% by weight of the graft shell (c₂).10. The molding material defined in claim 1, wherein the second graftpolymer (C) comprises, based on (C), from 40 to 75% by weight of therubber-elastic graft core (c₁), and from 25 to 60% by weight of thegraft shell (c₂).
 11. The molding material defined in claim 1, whereinthe rubber-elastic graft core (c₁) consists of butadiene or isoprene ormixtures thereof.
 12. A thermoplastic molding material containing, basedon the total weight of the molding material, A) from 72 to 88.5% byweight of a thermoplastic polymer of, based on A), a₁) from 50 to 100%by weight of styrene or α-methylstyrene or mixtures thereof, a₂) from 0to 50% by weight of acrylonitrile and a₃) from 0 to 50% by weight of oneor more further monoethylenically unsaturated monomers, B) from 10 to20% by weight of a first graft polymer having an average particlediameter d₅₀ from 400 to 600 nm, consisting of, based on B), b1) from 30to 90% by weight of a rubber-elastic graft core of, based on b1), b₁₁)from 80 to 99.99% by weight of n-butyl acrylate, b₁₂) from 0.01 to 20%by weight of at least one crosslinking monomer selected from the groupconsisting of dihydrodicyclopentadienyl acrylate, allyl esters ofacrylic acid and ally esters of methacrylic acid, and, b₁₃) from 0 to40% by weight of one or more further monoethylenically unsaturatedmonomers, and b₂) from 10 to 70% by weight of a graft shell of, based onb₂), b₂₁) from 50 to 100% by weight of styrene or α-methylstyrene ormixtures thereof, b₂₂) from 0 to 50% by weight of acrylonitrile, andb₂₃) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers, C) from 1.5 to 3% by weight of a second graftpolymer having a bimodal particle size distribution, the averageparticle diameter d₅₀ being from 25 to 200 nm on the one hand and from350 to 550 nm on the other hand, comprising, based on C), c₁) from 30 to90% by weight of a rubber-elastic graft core of, based on c₁), c₁₁) from50 to 100% by weight of butadiene or isoprene or mixtures thereof, and,c₁₂) from 0 to 50% by weight of one or more further monoethylenicallyunsaturated monomers, and, c₂) from 10 to 70% by weight of a graft shellof, based on c₂), c₂₁) from 50 to 100% by weight of styrene orα-methylstyrene or mixtures thereof, c₂₂) from 0 to 50% by weight ofacrylonitrile and, c₂₃) from 0 to 50% by weight of one or more furthermonoethylenically unsaturated monomers and D) from 0 to 50% by weight oflubricants or mold release agents, pigments, dyes, flameproofing agents,antioxidants, light stabilizers, fibrous or particulate fillers orreinforcing materials or antistatic agents or other additives, ormixtures thereof, the sum of the graft polymers B) and C) accounting forfrom 11.5 to 25% by weight of the sum of the three components A), B) andC), and the sum of the four components A) to D) being 100% by weight.13. A housing part or front panel for household appliances, a panel,switch or cover in the electrical sector or an installation box foroutdoor electrical apparatuses obtained from a thermoplastic moldingmaterial as claimed in claim
 12. 14. The molding material defined inclaim 12, wherein the amount of the thermoplastic polymer (A) is of from73 to 85% by weight, based on the sum of the four components A) to D).15. The molding material defined in claim 12, wherein the amount of thefirst graft polymer (B) is of from 12 to 18% by weight, based on the sumof the four components A) to D).
 16. The molding material defined inclaim 12, wherein the sum of the graft copolymers (B) and (C) amount tofrom 15 to 24% by weight, based on the sum of components (A) to (C). 17.The molding material defined in claim 12, wherein the sum of the graftcopolymers (B) and (C) amount to from 16 to 22% by weight, based on thesum of components (A) to (C).
 18. The molding material defined in claim12, wherein the thermoplastic polymer (A) consists of a₁) from 55 to 95%by weight of styrene or α-methylstyrene or mixtures thereof, a₂) from 5to 45% by weight of acrylonitrile and a₃) from 0 to 40% by weight of oneor more further monoethylenically unsaturated monomers.
 19. The moldingmaterial defined in claim 12, wherein the second graft polymer (C)comprises, based on (C), from 35 to 85% by weight of the rubber-elasticgraft core (c₁), and from 15 to 65% by weight of the graft shell (c₂).20. The molding material defined in claim 12, wherein the second graftpolymer (C) comprises, based on (C), from 40 to 75% by weight of therubber-elastic graft core (c₁), and from 25 to 60% by weight of thegraft shell (c₂).